Study On The Stability Analysis Method Of Freeze-thaw Rock Slope Based On Nonlinear Parameter Reduction

Centering on the strategic need of ensuring national resource security,supporting service to improve energy structure and safety,and promoting ecological civilization construction,the basic pattern of“attaching equal importance to both eastern and western China and focusing on western mineral resources”has already formed.Moderate development of resources in the alpine region has become an important national strategy for future resource development.Large number of major strategic projects in the alpine region have been sequentially proposed and put into practice,such as the well-known China Western Development.Meanwhile,the stability evaluation of freezing-thawing rock mass,especially the evaluation of freezing-thawing rock slope stability,has also become a key technical problem to ensure the construction and safety of engineering in the alpine regions.Therefore,supported by the National Natural Science Foundation of China（51774322）,in view of the essential characteristics of properties deterioration and fragile stability of freezing-thawing rock mass of alpine slopes,theoretical analysis,laboratory test,and numerical simulation were adopted to conduct relevant investigations on the freezing-thawing slope stability evaluation method in alpine region,mainly including:（1）Study on the Hoek-Brown parameter estimation method based on direct shear testThe shear expression of the nonlinear Hoek-Brown strength criterion was derived theoretically by simplifying the effect of the Hoek-Brown parameter s for the highly fractured rock mass,namely,the rock mass with GSI>50.Monte-Carlo method was used to generate a simplified discrete fracture network at laboratory test size.The fractured rock samples were prepared for direct shear experiments under multistage normal loads,and the corresponding shear strength was obtained,which were then input into MATLAB to fit the values of Hoek-Brown parameters mi and GSI.The calculated values of shear strength by fitted Hoek-Brown parameters of the samples under other normal stresses were compared with the experimental values,and the correctness of the shear expression of the nonlinear Hoek-Brown strength criterion was verified,which provided a new experimental way for determining Hoek-Brown parameters with simple experimental operation,easy sample preparation and convenient calculation.（2）Study on the decay law of Hoek-Brown parameters of rock mass under freezing-thawing cyclesThe freezing-thawing cycle experiments were designed to analyze the effect of the freezing-thawing cycle on the apparent damage and microcrack development of fractured samples.Through direct shear experiments and digital speckle technology,it is found that new cracks mainly initiate near the tip of prefabricated fractures,and continue to expand and connect with the increase of shear load.The crack strikes along the shear stress direction.According to the shear stress-shear displacement curves of the samples after different freezing-thawing cycles,the shear strength decreases with the increase of freezing-thawing cycles,and the stress drop rate in post-peak becomes slower.Subsequently,the values of Hoek-Brown parameters mi and GSI corresponding to random fractured rock samples under different freezing-thawing cycles were obtained by the same algorithm.Both decrease with the increase of freezing-thawing cycles,and the decreasing rate is also decreasing.The relationships between mi and GSI and freezing-thawing cycles N were obtained by fitting,and the values of mi and GSI under other freezing-thawing cycles could be roughly calculated.（3）Study on the slope stability analysis method based on nonlinear parameter reductionTo achieve the nonlinear reduction,the simple linear softening model was used to describe the different decay rates of cohesion and internal friction angle,and their reduction coefficients were derived.When the experimental conditions are not available,the shortest reduction path method can also be used to determine the reduction coefficients.Then,the slope equivalent influence angle was introduced,its variation law under different slope conditions was also analyzed,and the rapid determination method of slope equivalent influence angle was proposed.According to the difference between the slope equivalent influence angle and the actual slope angle,the contribution proportion of the two reduction coefficients in the comprehensive stability can be reflected,and the expression of the comprehensive safety factor can be established.Three known slope examples covering the unstable,critical,and stable slope states,including the cases that the slope equivalent influence angle is greater than or less than the actual slope angle,were used to calculate the safety factor.Through comparing with other existing methods,it is found that the proposed method can obtain the comprehensive safety factor of slope with sufficient accuracy within the allowable error range,which indicates that it has wide applicability.（4）Study on the slope stability analysis method based on Hoek-Brown parametric nonlinear reductionBased on the gravity loading method,the Hoek-Brown parameters and the equivalent Mohr-Coulomb parameters obtained by the global linear method were respectively assigned to the slope model for safety factor calculation under the same working conditions.The results show a good consistency only in the case of critical slope safety state.Therefore,a new slope stability calculation method based on Hoek-Brown parametric nonlinear reduction was proposed.According to the classical definition of slope safety factor,the function of slope safety factor with regard to rock mass compressive strengthσ_cmassand Hoek-Brown parameter m_b was deduced theoretically,and the expressions of slope critical curveσ_cmass-m_bunder different slope angle and unit weight were obtained by fitting.Combined with the shortest reduction path theory,the reduction coefficient relationship betweenσ_cmassand m_b was determined.Considering the difference of horizontal displacements of critical slope nodes,an algorithm for accurate identification and automatic extraction of slip lines was developed,and the safety factor of sliding surface was calculated to realize nonlinear reduction of slope strength of Hoek-Brown material.The slope calculation examples in the international publications were selected to verify the proposed reduction method,and the comparison of safety factors with other reduction methods proves the effectiveness and feasibility of this method.（5）Study on the decay law of slope stability under freezing-thawing effectNumerical models of fractured rock mass with different scales were constructed,and direct shear experiments were conducted to simulate the variation of shear strength with the model scale.Based on the concept of characteristic scale,the Hoek-Brown parameters of slope in rock mass scale were obtained.Combined with the influence of freezing-thawing effect on the strength of fractured rock mass,the strength parameters of slope under different freezing-thawing cycles were calculated.A numerical model of freezing-thawing slope was built,and the parameters after different freezing-thawing cycles were assigned to the rock mass.The stability analysis was carried out with the Hoek-Brown parameters nonlinear reduction method.The safety factor decreases with the increase of freeze-thaw cycles.At the same time,the sliding range of slope model shrinks to the freezing-thawing layer,and the instability of slope model tends to occur in the shallow rock mass.The sliding surface is tangent to the interface of the freezing-thawing layer.